Input voltage dependent control for active clamp flyback

ABSTRACT

A method for controlling a power converter includes turning on a power switch to maintain a desired output voltage, wherein the power switch is coupled to a primary winding to control a primary current flow and the output voltage is provided by a secondary winding. The method also includes adding a capacitance in parallel to the power switch at a time determined by a magnitude of an input voltage to the power converter. Here, the timing of adding the capacitance depends on the operation mode of the power controller. In a low input voltage mode, the capacitance is added in a demagnetization-time during which the secondary winding discharges. In a high input voltage mode, the capacitance is added in a discontinuous time.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 17/066,088, filed Oct. 8, 2020, entitled “INPUT VOLTAGEADAPTIVE JITTER IN POWER CONVERTER,” which is a continuation-in-part ofU.S. patent application Ser. No. 16/667,585, filed Oct. 29, 2019,entitled “CIRCUIT AND METHOD FOR JITTER GENERATION IN QUASI-RESONANTCONVERTER, which claims priority to U.S. Provisional Patent ApplicationNo. 62/816,048, filed Mar. 8, 2019, entitled “CIRCUIT AND METHOD FORJITTER GENERATION IN QUASI-RESONANT CONVERTER.” U.S. patent applicationSer. No. 17/066,088 also claims priority to U.S. Provisional PatentApplication No. 62/986,449, filed Mar. 6, 2020, entitled “INPUT VOLTAGEADAPTIVE JITTER FOR QUASI-RESONANT CONTROL.” The content of all theabove-listed applications is incorporated by reference in their entiretyherein for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to switching mode power supplies. Moreparticularly, the invention provides methods and apparatus for reducingelectromagnetic interference (EMI) of switching mode power supplies.

Regulated power supplies are indispensable in modern electronics. Forexample, the power supply in a personal computer often needs to receivepower input from various outlets. Desktop and laptop computers oftenhave regulated power supplies on the motherboard to supply power to theCPU, memories, and periphery circuitry. Regulated power supplies arealso used in a wide variety of applications, such as home appliances,automobiles, and portable chargers for mobile electronic devices, etc.

In general, a power supply can be regulated using a linear regulator ora switching mode controller. A linear regulator maintains the desiredoutput voltage by dissipating excess power. In contrast, a switchingmode controller rapidly switches a power transistor on and off with avariable duty cycle or variable frequency and provides an average outputthat is the desired output voltage.

In such a switched mode power supply system, a switch is connected tothe primary winding of the transformer. In the switching power supplies,the power transistor switches on and off periodically to convert theprimary current of the transformer to the secondary side. The stableoutput voltage will be obtained by regulating the duty cycle orfrequency of the primary side switch. Magnetic energy is stored in theinductance of the primary winding when the switch is turned on, and theenergy is transferred to the secondary winding when the switch is turnedoff. The energy transfer results in a current flowing through thesecondary winding and the rectifying diode. When the energy transfer iscompleted, the current stops flowing through the diode. If the switchingmode power supply, also referred to as the flyback converter, operatesin discontinuous conduction mode (DCM), during the discontinuous time, aresonant waveform of substantially sinusoidal oscillation of decreasingamplitude appears at the secondary winding and across the power switchdue to the built-in inductance and capacitance in the converter. Afterthe discontinuous time, the power switch is turned on again in the nextswitching cycle.

In quasi-resonant (QR) switching, the controller waits for one of thevalleys in the resonant waveform of the drain voltage and then turns onthe power switch. Compared with the traditional continuous conductionmode (CCM) and discontinuous conduction mode (DCM) of operation in aflyback converter, quasi-resonant switching can reduce turn-on losses atthe power switch, thus increasing efficiency and lowering devicetemperatures.

Compared with linear regulators, switching mode power supplies have theadvantages of smaller size, higher efficiency and larger output powercapability. On the other hand, they also have the disadvantages ofgreater noise, especially electromagnetic Interference (EMI) at thepower transistor's switching frequency or its harmonics.

EMI is a critical issue in the design of a switching mode power supply.In order to reduce EMI, different frequency jittering techniques can beused. For example, switching frequencies may be varied by frequencymodulation in order to spread out the electromagnetic radiation energyacross a frequency range. One way to vary the switching frequency is toadd a jitter component to the system clock. This technique helpsreducing average EMI emission. However, implementing effective jitteringcan be difficult in a quasi-resonant (QR) converter, as explainedfurther below.

BRIEF SUMMARY OF THE INVENTION

The inventor has recognized that, in a quasi-resonant (QR) converter, itis difficult to implement effective frequency jitter. Underdiscontinuous conduction mode (DCM), the flyback converter has an LCresonant waveform during the discontinuous time after the secondary sidecurrent is discharged. The QR operation turns on the power switch at avalley point of resonant voltage. The turn-on condition at a valleypoint of the resonant voltage can limit the switching frequencies of theflyback system and prevent the switching frequency to spread over arelatively large frequency range. As a result, the effectiveness of thejittering is limited.

This invention teaches a technique for introducing jitter in theswitching frequency of a power converter. The power converter includes apower switch controlling the primary current flow, and a time-varyingcapacitance is coupled in parallel to the power switch. The time-varyingcapacitance adds a frequency jitter to the frequency of the converter.

This invention also teaches a power converter having a power switchcontrolling current flow in the power converter and a variablecapacitance coupled in parallel to the power switch. The variablecapacitance is configured to add a frequency jitter to the powerconverter. In some embodiments, the frequency jitter comprises a firstportion that varies with an input voltage of the power converter and asecond portion that is a time-varying function. The first portion thatvaries with an input voltage can reduce the switching loss of the powerconverter. In some embodiments, the variable capacitance comprises atransistor coupled in series with a capacitor.

As an example, this invention teaches a power converter that includes atransformer having a primary winding for coupling to an input voltage, asecondary winding providing an output voltage of the power converter,and a sensing circuit for providing a sensing signal, which monitors aresonant waveform of the power converter during discontinuous time. Thepower converter includes a power switch for coupling to the primarywinding of the power converter to control a primary current flow, and acapacitor and a modulation switch coupled in parallel to the powerswitch.

The power converter can also include a control circuit that includes apower controller for turning on the power switch in response to theresonant waveform in the sensing signal during a discontinuous time, anda jitter controller for turning on the modulation switch in response tothe resonant waveform in the sensing signal during the discontinuoustime. The jitter controller varies a turn-on time of the modulationswitch to add a frequency jitter to a switching frequency of the powerconverter. The frequency jitter can include a first portion that varieswith the input voltage of the power converter and a second portion thatis a time-varying function.

As another example, this invention teaches a method for controlling apower converter. The method includes turning on a power switch inresponse to a resonant waveform in a sensing signal during adiscontinuous time of the power converter, wherein the power switch iscoupled to a primary winding of the power converter to control a primarycurrent flow. The method also includes adding a capacitance in parallelto the power switch during the discontinuous time, to vary anoscillation period of the resonant waveform to add a frequency jitter toa switching frequency of the power converter. The method can add acapacitance in parallel to the power switch by turning on a modulationswitch that is coupled in series with a capacitor, wherein the capacitorand the modulation switch are coupled in parallel with the power switch.The frequency jitter can include a first portion that varies with aninput voltage of the power converter and a second portion that is atime-varying function.

The invention also teaches a power converter having a power switchcontrolling current flow in the power converter and a variablecapacitance coupled in parallel to the power switch. The variablecapacitance is configured to add a frequency jitter to the powerconverter. In some embodiments, the frequency jitter comprises a firstportion that varies with an input voltage of the power converter and asecond portion that is a time-varying function. In some embodiments, thevariable capacitance comprises a transistor couple in series with acapacitor.

In another example, this invention teaches a method for controlling aquasi-resonant (QR) converter to reduce electromagnetic interference(EMI). The converter includes a power switch coupled to a primarywinding of the converter to control a primary current flow, and asensing signal monitoring the converter through an auxiliary winding.The method includes turning on the power switch at a valley point of aresonant waveform in the sensing signal during a discontinuous time ofthe converter for quasi-resonant (QR) operation. The method furtherincludes adding a capacitance in parallel to the power switch at a peakpoint of the resonant waveform in the sensing signal during thediscontinuous time, to vary an oscillation period of the resonantwaveform, which leads to variations of the discontinuous time andchanges the switching frequency of the converter. A modulation switchwith a time-varying on-time can be used to control the duration of timein which the additional capacitance is in effect. For example, thetime-varying duration can be a linear function of time to spread out theswitching frequency across a relatively large frequency range.

In another example, this invention teaches a control circuit for aquasi-resonant (QR) converter. The control circuit includes aquasi-resonant controller for turning on a power switch at a valleypoint of a resonant waveform in a sensing signal during a discontinuoustime of the converter. The power switch is coupled to a primary windingof the converter to control a primary current flow, and the sensingsignal is monitoring the resonant waveform of the converter through anauxiliary winding. The control circuit also includes a jitter controllerfor adding a capacitance in parallel to the power switch at a peak pointof the resonant waveform in the sensing signal during the discontinuoustime. The jitter controller varies an oscillation period of the resonantwaveform to add a frequency jitter to a switching frequency of theconverter.

In another example, this invention teaches a quasi-resonant (QR)converter. The converter includes a transformer having a primary windingfor coupling to an external input voltage, a secondary winding providingan output voltage of the converter, and an auxiliary winding forproviding a sensing signal of the converter. The converter also has apower switch for coupling to the primary winding of the converter tocontrol a primary current flow, and a capacitor and a modulation switchcoupled in parallel to the power switch, with the modulation switchcoupled in series with the capacitor. The converter also has a controlcircuit that includes a quasi-resonant controller and a jittercontroller. The quasi-resonant controller turns on the power switch at avalley point of a resonant waveform in the sensing signal during adiscontinuous time. The jitter controller turns on the modulation switchat a peak point of the resonant waveform in the sensing signal duringthe discontinuous time. The jitter controller varies a turn-on time ofthe modulation switch to add a frequency jitter to a switching frequencyof the converter.

In another example, this invention teaches a control circuit for aquasi-resonant (QR) converter. The control circuit includes aquasi-resonant controller for turning on a power switch at a valleypoint of a resonant waveform in a sensing signal during a discontinuoustime of the converter, wherein the power switch is coupled to a primarywinding of the converter to control a primary current flow, and thesensing signal monitors the resonant waveform of the converter throughan auxiliary winding. The control circuit also includes a jittercontroller for modulating a capacitance coupled in parallel to the powerswitch to vary an oscillation period of the resonant waveform to add afrequency jitter to a switching frequency of the converter. Thefrequency jitter includes a first portion that varies with an inputvoltage of the converter and a second portion that is a time-varyingfunction. In an embodiment, the jitter controller starts varying theoscillation period at a peak point of the resonant waveform in thesensing signal during the discontinuous time.

In another example, this invention teaches a method for controlling aquasi-resonant (QR) converter. The method includes turning on a powerswitch at a valley point of a resonant waveform in a sensing signalduring a discontinuous time of the converter. The power switch iscoupled to a primary winding of the converter to control a primarycurrent flow, and the sensing signal monitors the resonant waveform ofthe converter through an auxiliary winding. The method also includesmodulating a capacitance coupled in parallel to the power switch to varyan oscillation period of the resonant waveform to add a frequency jitterto a switching frequency of the converter. The oscillation periodincludes an offset that varies with an input voltage of the converterand a time-varying dithering period. In an embodiment, the method alsoincludes starting to vary the oscillation period at a peak point of theresonant waveform in the sensing signal during the discontinuous time.

In a flyback converter, to reduce the switching loss, a modulationswitch Q_(M) can be turned on to connect a modulation capacitor C_(SN)in parallel with the capacitance C_(OSS) of the power switch forinducing a larger negative resonant current on Lm. However, in order toprecisely control the negative current on Lm, the control circuit needsto accurately detect the peak of the resonant waveform for turning onthe Q_(M). As a result, the transformer must be operated underdiscontinuous conduction mode (DCM).

An aspect of this invention teaches to change the operating mode of theflyback system according to the input voltage. The efficiency of aflyback system is determined by different factors depending on the inputvoltage. The efficiency of a flyback system is often dominated byswitching loss under high input voltage, and can be dominated byconduction loss under low input voltage. By changing the operating mode,the flyback system can benefit from lower peak current under low inputvoltage and can also benefit from drain-source voltage V_(DS) reductionunder high input voltage.

In an example, this invention teaches a power converter that includes atransformer having a primary winding for coupling to an input voltage, asecondary winding providing an output voltage of the power converter,and a sensing circuit for providing a sensing signal monitoring theconverter through an auxiliary winding. The power converter alsoincludes a power switch for coupling to the primary winding of the powerconverter to control a primary current flow, a capacitor and amodulation switch coupled in parallel to the power switch, themodulation switch being coupled in series with the capacitor, and acontrol circuit. The control circuit includes a power controller forturning on the power switch to maintain a desired output voltage and amode selection circuit for comparing the input voltage with a referencevoltage and providing a mode selection signal that indicates a highinput voltage mode if the input voltage is higher than the referencevoltage and a low input voltage mode if the input voltage is lower thanthe reference voltage. The control circuit further includes a modulationcontroller configured to turn on the modulation switch to activate thecapacitor according to the mode selection signal. In the low inputvoltage mode, the modulation switch is turned on in ademagnetization-time during which the secondary winding discharges. Inthe high input voltage mode, the modulation switch is turned on in adiscontinuous time during which no current flows in the transformer andthe sensing signal is characterized by a resonant waveform.

In another example, this invention teaches a control circuit for a powerconverter. The controller includes a power controller for turning on apower switch to maintain a desired output voltage. The power switch iscoupled to a primary winding of the power converter to control a primarycurrent flow. The controller also includes a modulation controllerconfigured to activate a modulation capacitance coupled in parallel tothe power switch. In a low input voltage mode, the modulation switch isturned on in a demagnetization-time during which the secondary windingdischarges. In a high input voltage mode, the modulation switch isturned on in a discontinuous time.

In another example, the invention teaches a method for controlling apower converter. The method includes turning on a power switch tomaintain a desired output voltage, wherein the power switch is coupledto a primary winding to control a primary current flow and the outputvoltage is provided by a secondary winding. The method also includesadding a capacitance in parallel to the power switch at a timedetermined by a magnitude of an input voltage to the power converter. Ina low input voltage mode, the capacitance is added in ademagnetization-time during which the secondary winding discharges. In ahigh input voltage mode, the capacitance is added in a discontinuoustime.

Definitions

The terms used in this disclosure generally have their ordinary meaningsin the art within the context of the invention. Certain terms arediscussed below to provide additional guidance to the practitionersregarding the description of the invention. It will be appreciated thatthe same thing may be said in more than one way. Consequently,alternative language and synonyms may be used.

A power switch as used herein refers to a semiconductor switch, forexample, a transistor, that is designed to handle high power levels.

A power MOSFET is a specific type of metal oxide semiconductorfield-effect transistor (MOSFET) designed to handle significant powerlevels. An example of a power MOSFET for switching operations is calleda double-diffused MOS or simply DMOS.

A body diode in a power MOSFET is formed when the body and source arecoupled together, and the body diode is formed between drain and source.The diode is located between the drain (cathode) and the source (anode)of the MOSFET making it able to block current in only one direction.

A power converter is an electrical or electro-mechanical device forconverting electrical energy, such as converting between AC and DC orchanging the voltage, current, or frequency, or some combinations ofthese conversions. A power converter often includes voltage regulation.

A regulator or voltage regulator is a device for automaticallymaintaining a constant voltage level.

A switching regulator, or switch mode power supply (SMPS) is a powerconverter that uses an active device that switches on and off tomaintain an average value of output. In contrast, a linear regulator ismade to act like a variable resistor, continuously adjusting a voltagedivider network to maintain a constant output voltage, and continuallydissipating power.

Continuous conduction mode (CCM) is an operational mode of a powerconverter, in which the system turns on the primary side current beforethe secondary side current is stopped.

Discontinuous conduction mode (DCM) is an operational mode of a powerconverter, in which there exists a discontinuous time period, duringwhich the current flow is stopped on both the primary side and thesecondary side. The primary side is turned on again following thediscontinuous time period.

Quasi-resonant (QR) mode is an operational mode of a power converteroperating in discontinuous conduction mode (DCM), in which the primaryside is turned on at a valley point of a resonant waveform during thediscontinuous time period. Quasi-resonant operation can reduce switchingloss of the power converter.

An operational amplifier (op-amp or opamp) refers to a DC-coupledhigh-gain electronic voltage amplifier with a differential input and,usually, a single-ended output. An operational amplifier can becharacterized by a high input impedance and a low output impedance, andcan be used to perform mathematical operations in analog circuits.

A voltage reference is an electronic device that ideally produces afixed (constant) voltage irrespective of the loading on the device,power supply variations, temperature changes, and the passage of time.

A reference voltage is a voltage value that is used as a target for acomparison operation.

When the term “the same” is used to describe two quantities, it meansthat the values of two quantities are determined the same withinmeasurement limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a flyback converter thatembodies certain aspects of this invention;

FIG. 2A is a simplified schematic diagram of part of flyback converter100 of FIG. 1, and FIG. 2B shows waveform diagrams that illustrate aquasi-resonant operation of the converter that embodies certain aspectsof this invention;

FIG. 3 is a waveform diagram that illustrates a jitter in aquasi-resonant operation of the converter that embodies certain aspectsof this invention;

FIGS. 4A-4D illustrate an example of providing jitter to a converterthat that embodies certain aspects of this invention;

FIG. 5A illustrates the waveforms for the QR converter in which thejitter time t_(jitter) is near zero, and the oscillation period for aresonant waveform is T_(QR), as described above;

FIG. 5B illustrates the waveforms for the QR converter in which thejitter time t_(jitter) is at the maximum, t_(jitterMax), and theoscillation period for a resonant waveform is T_(QR1), as describedabove;

FIGS. 6-10 are schematic or waveform diagrams illustrating thestructures and functions of various components in the control circuit ina converter that embody certain aspects of this invention;

FIG. 6 shows schematic and waveform diagrams illustrating the structuresand functions of a valley detector and a peak detector in the controlcircuit in a converter that embodies certain aspects of this invention;

FIG. 7A is a simplified schematic diagram illustrating a jittercontroller, and FIG. 7B illustrates a waveform diagram that describesthe operation of the jitter controller that embodies certain aspects ofthis invention;

FIG. 8A is a simplified schematic diagram illustrating a blanking timegeneration circuit, and FIG. 8B illustrates a waveform diagram thatdescribes the operation of the blanking time generation circuit thatembodies certain aspects of this invention;

FIG. 9 is a simplified schematic diagram illustrating a first example ofa quasi-resonant controller that embodies certain aspects of thisinvention;

FIG. 10 is a simplified schematic diagram illustrating a second exampleof a quasi-resonant controller that embodies certain aspects of thisinvention;

FIG. 11A is a simplified schematic diagram illustrating a quasi-resonant(QR) converter with jitter that embodies certain aspects of thisinvention;

FIG. 11B is a waveform diagram illustrating the operation of thequasi-resonant (QR) converter with jitter of FIG. 11B that embodiescertain aspects of this invention;

FIG. 12A is a simplified schematic diagram illustrating a quasi-resonant(QR) converter with jitter that embodies certain aspects of thisinvention;

FIG. 12B is a waveform diagram illustrating the operation of thequasi-resonant (QR) converter with jitter of FIG. 12A that embodiescertain aspects of this invention;

FIG. 13 is a simplified flowchart that illustrates a method forcontrolling a quasi-resonant (QR) converter that embodies certainaspects of this invention;

FIG. 14 is a simplified schematic diagram illustrating a power converterwith jitter that embodies certain aspects of this invention;

FIG. 15 illustrates the waveforms for the power converter of FIG. 14that embodies certain aspects of this invention;

FIG. 16 illustrates a method of providing a range of frequency jitter toa converter that that embodies certain aspects of this invention;

FIG. 17 illustrates waveforms for a converter that embodies certainaspects of this invention;

FIG. 18 illustrates waveforms for a converter that embodies certainaspects of this invention;

FIGS. 19A-19C illustrate waveforms for the converter that embodiescertain aspects of this invention;

FIG. 20 illustrates waveforms for a converter that embodies certainaspects of this invention;

FIG. 21 is a simplified schematic diagram of a flyback converter thatembodies certain aspects of this invention;

FIG. 22 shows a simplified schematic and waveform diagrams illustratingthe structures and functions of a valley detector and a peak detector inthe control circuit in a converter that embodies certain aspects of thisinvention;

FIG. 23 shows a simplified schematic for detecting the input voltagethat embodies certain aspects of this invention;

FIG. 24 is a simplified schematic diagram illustrating a jittercontroller that embodies certain aspects of this invention;

FIG. 25 illustrates a waveform diagram that describes the operation ofthe jitter controller of FIG. 24 that embodies certain aspects of thisinvention;

FIG. 26 is a simplified schematic diagram illustrating a blanking timegeneration circuit and waveform diagram that embodies certain aspects ofthis invention;

FIG. 27 is a simplified schematic diagram illustrating an example of apower controller that embodies certain aspects of this invention;

FIG. 28 is a simplified flowchart that illustrates a method forcontrolling a power converter that embodies certain aspects of thisinvention;

FIG. 29 is a simplified schematic diagram and a waveform diagramillustrating an example of a flyback convertor that embodies certainaspects of this invention;

FIG. 30 illustrates discontinuous conduction mode (DCM) waveforms offlyback convertor 2900 of FIG. 29 operated by jitter controller thatembodies certain aspects of this invention;

FIG. 31 illustrates the quasi-resonant (QR) waveforms of flybackconvertor 2900 of FIG. 29 operated under low input voltage mode thatembodies certain aspects of this invention;

FIG. 32 illustrates the continuous conduction mode (CCM) waveforms offlyback convertor 2900 of FIG. 29 operated under low input voltage modethat embodies certain aspects of this invention;

FIG. 33 illustrates the waveforms of flyback convertor 2900 of FIG. 29operated under high input voltage mode that embodies certain aspects ofthis invention;

FIG. 34 is a simplified schematic diagram of a power converter thatembodies certain aspects of this invention;

FIG. 35 is a simplified schematic diagram for a mode selector in powerconverter 3400 of FIG. 34 that embodies certain aspects of thisinvention;

FIG. 36 is a simplified schematic diagram for an alternative modeselector in power converter 3400 of FIG. 34 that embodies certainaspects of this invention;

FIG. 37 is a simplified schematic diagram for an example of the Q_(M)controller in power converter 3400 of FIG. 34 that embodies certainaspects of this invention;

FIG. 38 shows a portion of Q_(M) controller 3700 of FIG. 37 and signalwaveforms for operation in low input voltage mode that embodies certainaspects of this invention;

FIG. 39 shows another portion of Q_(M) controller 3700 of FIG. 37 andsignal waveforms for operation in high input voltage mode that embodiescertain aspects of this invention;

FIG. 40A is a simplified block diagram of an alternative Q_(M)controller in power converter 3400 of FIG. 34, and FIG. 40B illustratescorresponding signal waveforms for operation in low input voltage modethat embodies certain aspects of this invention;

FIG. 41 illustrates a simplified block diagram of a power controller andits operating waveforms in different operating mode that embodiescertain aspects of this invention;

FIG. 42 illustrates a simplified block diagram of another example apower controller and its operating waveforms in different operating modethat embodies certain aspects of this invention;

FIG. 43 illustrates a simplified block diagram of an oscillator blockthat embodies certain aspects of this invention;

FIG. 44 illustrates a simplified block diagram of the peak detector andvalley detector and the associated waveforms that embodies certainaspects of this invention;

FIG. 45 illustrates a simplified block diagram of an embodiment of aQ_(M) on-time calculator and the associated waveforms that embodiescertain aspects of this invention;

FIG. 46 illustrates a simplified block diagram of another embodiment ofa Q_(M) on-time calculator and the associated waveforms that embodiescertain aspects of this invention; and

FIG. 47 is a simplified flowchart that illustrates a method forcontrolling a power converter that embodies certain aspects of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified schematic diagram of a flyback converter thatembodies certain aspects of this invention. Flyback converter 100includes a transformer 102, which includes a primary winding 141 forcoupling to an input voltage V_(in) and a secondary winding 142 forproviding the output voltage V_(out) through a rectifying diode 120 anda capacitor 119. A current in the rectifying diode is I_(D). Transformer102 also has an auxiliary winding 143 for providing a sensing signalDMEG monitoring a resonant waveform of the converter during adiscontinuous time. Auxiliary winding 143 also provides a voltageV_(cc), which can be used as a power supply for the support circuitry,such as a control circuit. In FIG. 1, Vp denotes the voltage at theprimary winding, Vs denotes the voltage at the secondary winding, and Vadenotes the voltage at the auxiliary winding. In FIG. 1, L_(m)represents the inductance of the primary winding, and N represents theratio of coil turns in the primary winding to the secondary winding.C_(oss) represents the capacitance associated with primary switch 101,including the capacitance from the body diode. Vds represents thedrain-source voltage across the primary switch.

Converter 100 includes a control circuit 150. Power supply 100 alsoincludes a power switch 101 (also designated as Q_(L)) coupled toprimary winding 141 and control circuit 150 for receiving a controlsignal to turn on and off power switch 101 to control the primarycurrent through primary winding 141 in order to regulate output voltageV_(OUT). In FIG. 1, power switch 101 is shown as a MOSFET powertransistor. In the embodiment of FIG. 1, control circuit 150 can receiveits operating power from V_(cc) provided by the auxiliary winding.Control circuit 150 can also receive sensing signal DMEG from auxiliarywinding 143. Control circuit 150 can also receive a current sense signalVcs representative of the primary current through a current senseresistor 116 with a resistance Rcs. Control circuit 150 provides acontrol signal Drive to control power switch 101. Control circuit 150turns on the power switch 101 based on information provided by thesensing signal DMEG and turns off the power 101 based on informationprovided by the current sense signal Vcs.

As shown in FIG. 1, converter 100 also has a modulation capacitor 130(also designated as C_(sn)) and a modulation switch 131 (also designatedas Q_(M)) coupled in parallel to power switch 101. In this arrangement,modulation switch 131 is coupled in series with capacitor 130. Controlcircuit 150 includes a valley detector 151 for detecting valley pointsin the resonant waveform in the sensing signal DMEG during thediscontinuous time of converter 100. Control circuit 150 also includes apeak detector 152 for detecting peak points in the resonant waveform inthe sensing signal during the discontinuous time. Control circuit 150also includes a quasi-resonant (QR) controller 153 for turning on thepower switch 101 at a valley point of a resonant waveform in the sensingsignal during a discontinuous time. Control circuit 150 further includesa jitter controller 154 for turning on the modulation switch 131 at apeak point of the resonant waveform in the sensing signal during thediscontinuous time. Control circuit 150 also includes an oscillator OSC155, which provides a T_(FMAX) signal to jitter controller 154. Inaddition to the T_(FMAX) signal, jitter controller 154 also receives aPpulse signal from peak detector 152, and provides a Mod_Off signal toQR controller 153. QR controller 153 also receives a Vpulse signal fromvalley detector 151. Control circuit 150 also includes a flipflop 156that outputs the Drive signal.

As explained in detail below, the jitter controller 154 varies a turn-ontime of the modulation switch 131 to add a frequency jitter to theswitching frequency of the converter. The waveform of Vds is mirrored tothe auxiliary winding and detected by a peak detector and a valleydetector. The jitter controller determines the target peak count to turnon Q_(M) and linearly modulates the on time of Q_(M). After Q_(M) isturned off, the QR controller is enabled by the jitter controller towait for proper valley count and turn on Q_(L) at the valley of Vds.

FIG. 2A is a simplified schematic diagram of part of flyback converter100 of FIG. 1, and FIG. 2B shows waveform diagrams that illustrate aquasi-resonant operation of the converter that embodies certain aspectsof this invention. In FIG. 2B, the following waveforms are shown over aswitch cycle T of the converter. The first waveform “Drive” shows thecontrol signal Drive applied to the primary switch 101. The secondwaveform “Vds” shows the drain-to-source voltage Vds of the primaryswitch, which can be reflected in the sensing signal DMEG of FIG. 1. Thethird waveform shows the primary current I_(PRI) feeding into theprimary side of the transformer. The fourth waveform shows the secondarycurrent I_(SEC) flowing out of the secondary side of the transformer. Ascan be seen in FIG. 2B, the primary switch is turned on during timeperiod t_(ON), and turned off during time period t_(OFF). During timeperiod t_(ON), the primary current flows through the primary switch. Thesecondary current flows during a first portion of time period t_(OFF).Time period t_(DIS) is a discontinuous time, during which both theprimary current and the secondary current are off. During discontinuoustime t_(DIS), a resonant waveform 220 exists in the waveforms of Vds, aswell as DMEG. In the example of FIG. 2B, resonant waveform 220 is shownto have three peak points 221, 223, and 225 and three valley points 222,224, and 226, marked by K=1, K=2, and K=3. In a quasi-resonantoperation, the quasi-resonant controller 153 in FIG. 1 turns on thepower switch at a valley point of a resonant waveform in the sensingsignal during the discontinuous time. In FIG. 2B, the primary switch isturned on at the third valley point (K=3) of resonant waveform 220.

In a quasi-resonant operation, the turn-on condition “valley of resonantvoltage” can improve power efficiency, but limits the switchingfrequency of the flyback system. According to FIG. 2, the equation ofpower delivery P_(OUT) in one switching cycle of the converter is:

$P_{OUT} = {\frac{E_{Lm}}{T} = \frac{\frac{1}{2}L_{m}I_{peak}^{2}}{{\frac{V_{IN} + {NV}_{OUT}}{L_{m}} \times I_{peak}} + {\left( {K - \frac{1}{2}} \right)T_{QR}}}}$$T = {{\frac{V_{IN} + {NV}_{OUT}}{L_{m}} \times I_{peak}} + {\left( {K - \frac{1}{2}} \right)T_{QR}}}$

where E_(Lm) is the energy stored in Lm, K is the valley count, andT_(QR) is an oscillation period of the resonant waveform. In oneswitching period T, the power convertor stores energy E_(Lm) into Lmduring t_(ON) and delivers the E_(Lm) to output during t_(OFF). Thus,the average transferred power P_(OUT) is equal to the E_(Lm) over T. Inthe P_(OUT) equation, the only variable that can be modulated is thepeak current I_(peak) because the valley count K should be a naturalnumber. However, a power convertor should deliver P_(OUT) equal to theload that application is required. The I_(peak) has only one solutionwith a specific number K, and the switching period T is a constant valuegiven by the couple of solution I_(peak) and K. As a result, anydisturbance of I_(peak) for dithering switching period will becompensated by an external feedback loop and the switching frequencywill converge to the given value which can fulfill the output loadP_(OUT).

The invention teaches a control circuit for a quasi-resonant (QR)converter. As shown in the converter 100 shown in FIGS. 1, 2A, and 2B,for quasi-resonant operation, control circuit 150 includes aquasi-resonant controller 153 for turning on a power switch 101 at avalley point 222, 224, or 226, etc., of a resonant waveform 220 in asensing signal DMEG during a discontinuous time t_(DIS) of theconverter. The power switch is coupled to a primary winding 141 of theconverter to control a primary switch, and the sensing signal DMEGmonitoring the resonant waveform through an auxiliary winding 143.Control circuit 150 also includes a jitter controller 154 for adding acapacitance 130 in parallel to the power switch 101 at a peak point ofthe resonant waveform 220 in the sensing signal DMEG during thediscontinuous time t_(DIS). For example, the capacitance can be added atpeak points such as peak points 221, 223, or 225, etc. By varying thelength of time during which the capacitance is coupled, the jittercontroller 154 can vary the oscillation period T_(QR) of the resonantwaveform to add a frequency jitter to the switching frequency of theconverter.

FIG. 3 is a waveform diagram that illustrates a jitter operation in aquasi-resonant operation of the converter that embodies certain aspectsof this invention. In FIG. 3, the following waveforms are shown over aswitch cycle T of the converter, with reference to FIGS. 1, 2A, and 2B.The top waveform Vds shows the drain-to-source voltage “Vds” of theprimary switch. The middle waveform “Drive” shows the control signalDrive applied to the primary switch 101. The bottom waveform “Mod” showsthe control signal Mod applied to the modulation switch 131. A resonantwaveform 220 exists in the waveforms of Vds during the discontinuoustime. In the example of FIG. 3, resonant waveform 320 is shown to havethree peak points 321, 323, and 325, and three valley points 322, 324,and 326, marked by K=1, K=2, and K=3. In the quasi-resonant operation,the quasi-resonant controller 153 in FIG. 1 turns on the power switch101 at a valley point 326 of the resonant waveform in the sensing signalduring the discontinuous time. In FIG. 3, the primary switch is turnedon at the third valley point 326 (K=3) of resonant waveform 320, at theend of the switching period T of the switching cycle.

In FIG. 3, the control signal Mod turns on the modulation switch 131 fora time period t_(jitter). During the time period t_(jitter), modulationswitch 131 is on, and modulation capacitor 130 is connected in parallelwith primary switch 101, causing a modulation capacitor C_(sn) to beconnected in parallel to the capacitance of the primary switch C_(oss).Accordingly, during the time period t_(jitter), the oscillation periodfor resonant waveform 320 is increased from T_(QR) to T_(QR1).

T _(QR)=2π√{square root over (L _(m) C _(OSS))}

T _(QR1)=2π√{square root over (L _(m)(C _(OSS) +C _(sn)))}

where L_(m) is the inductance of the primary winding, C_(oss) is thecapacitance associated with the primary switch, and C_(sn) is thecapacitance of the modulation capacitor. By varying the time periodduring which the modulation capacitor C_(sn) is connected, the period Tof the switching cycle of the converter is also changed.

In order to reduce the hard switching loss as turning on MOStransistors, the modulation switch Q_(M) should be turned on at a peakpoint of Vds, and the primary switch Q_(L) should be turned on at avalley point of Vds. To fulfill the turn on timings, peak and valleydetectors sense the DMEG node and produce pulse signals P_(Pulse) andV_(Pulse) based on the peaks and valleys of Vds sinusoid oscillation.

In order to introduce jitter into the switching cycle, the turn-on timet_(jitter) for modulation switch 131 is varied over time. FIGS. 4A-4Dillustrate an example of varying the turn-on time t_(jitter) thatembodies certain aspects of this invention. FIG. 4A shows that the timeperiod t_(jitter), the modulation switch on-time, is varied in a linearmanner between a minimum of 0 and a maximum of t_(jitterMax). FIG. 4Bshows a corresponding switch frequency variation over a range of Δf.FIG. 4C shows a corresponding variation in the switch frequencywaveform. FIG. 4D shows a spectrum plot of the Nth harmonic of theswitch frequency waveform, with a center frequency of Nf_(avg). Eventhough the example in FIGS. 4A-4D illustrates a linear function for thejitter frequency, other time-varying functions can also be used.

The maximum range of the frequency spectrum is dependent on the valleycount of the primary switch turn on and the value of modulationcapacitor C_(sn). If the modulation switch Q_(M) turn-on signal isapplied on the same valley count as the primary switch turn on, thedithering range of the total switching period will be only a half cycleof mixed LC resonant and can be expressed as follows.

$\mspace{20mu}{{{\frac{1}{2}{\left( {T_{QR1} - T_{QR}} \right).\Delta}\; T} \leq {\frac{1}{2}\left( {T_{QR1} - T_{QR}} \right)}} = {{\pi\sqrt{L_{m}C_{OSS}}\left( {\sqrt{1 + \frac{C_{sn}}{C_{OSS}}} - 1} \right)\mspace{14mu}{for}\mspace{14mu} K_{jitter}} = {K_{QR}.}}}$

FIGS. 5A and 5B illustrate the waveforms for the QR converter in whichthe jitter causes the oscillation period for the resonant waveform tovary. FIG. 5A illustrates the waveforms for the QR converter in whichthe jitter time t_(jitter) is nearly zero, and the oscillation periodfor resonant waveform is T_(QR), as described above. FIG. 5B illustratesthe waveforms for the QR converter in which the jitter time t_(jitter)is at the maximum, t_(jitterMax), and the oscillation period forresonant waveform is T_(QR1), as described above.

FIGS. 6-10 are schematic or waveform diagrams illustrating thestructures and functions of various components in the control circuit ina converter that embody certain aspects of this invention.

FIG. 6 shows schematic and waveform diagrams illustrating the structuresand functions of a valley detector and a peak detector in the controlcircuit in a converter that embodies certain aspects of this invention.In FIG. 6, a valley detector 610 includes a first comparator 611 forcomparing a sensing signal DMEG and a valley reference voltage Vref_V. Apeak detector 620 includes a second comparator 622 for comparing asensing signal DMEG and a peak reference voltage Vref_P. Waveform 630illustrates the sensing signal DMEG, with the valley reference voltageVref_V and peak reference voltage Vref_P. Waveform 640 illustrates theoutput waveform of the second comparator 622, showing a pulse signalP_(Pulse) where the sensing signal DMEG is greater than peak referencevoltage Vref_P. Waveform 650 illustrates the output waveform of thefirst comparator 611, showing a pulse signal V_(Pulse) where the sensingsignal DMEG is less than valley reference voltage Vref_V.

FIG. 7A is a simplified schematic diagram illustrating a jittercontroller, and FIG. 7B illustrates a waveform diagram that describesthe operation of the jitter controller that embodies certain aspects ofthis invention. As shown in FIGS. 7A and 7B, jitter controller 700 is anexample of a jitter controller that can be used as jitter controller 154in converter 100 in FIG. 1. Jitter controller 700 includes two Dflipflops. A first D flipflop 710 has a first input terminal 711 forreceiving a blanking time signal T_(FMAX), a second input terminal 712for receiving the peak pulse signal P_(Pulse), a first output terminal713 for providing a modulation-on signal Mod_On, and a second outputterminal 714 for providing a complement of the modulation on signal.Jitter controller 700 also includes a jitter duration circuit 720 havinga ramp signal circuit 721 with a current source 722 and capacitor 723that starts charging the capacitor upon receiving the complement of themodulation on signal, from the second output terminal 714 of the first Dflipflop 710, to produce a ramp signal RAMP. Jitter duration circuit 720also has a comparator 725 for comparing the ramp signal RAMP and ajitter reference voltage V_(jitter) and providing a jitter stop signal727. A second D flipflop 730 has a first input terminal 731 forreceiving logic high signal Logic H, a second input terminal 732 forreceiving the modulation on signal Mod_On from the first D flipflop 710,a reset terminal 735 for receiving the jitter stop signal 727, a firstoutput terminal 733 for providing a modulation switch turn-on signalMod, and a second output terminal 734 for providing a complement of themodulation switch turn-on signal. Jitter controller 700 also has an ANDcircuit 740 for receiving the modulation on signal Mod_On and thecomplement of the modulation switch turn-on signal Mod and providing amodulation switch turn-off signal Mod_Off. In FIG. 7B, TPON illustratesthe on-time of the modulation switch.

FIG. 8A is a simplified schematic diagram illustrating a blanking timegeneration circuit, and FIG. 8B illustrates a waveform diagram thatdescribes the operation of a blanking time generation circuit thatembodies certain aspects of this invention. Similar to jitter durationcircuit 720 in jitter controller 700, blanking time generation circuit800 includes a ramp signal circuit 810 with a current source andcapacitor that starts charging the capacitor upon receiving a Resetsignal to produce a ramp signal. As shown in FIG. 8A, the Reset signalis a pulse signal produced by a rising edge one-shot circuit 820 at therising edge of the primary switch Q_(L) turn-on signal. Blanking timegeneration circuit 800 also has a comparator 830 for comparing the rampsignal and a reference voltage V_(FMAX) and providing blanking timesignal T_(FMAX). FIG. 8B shows the waveforms for the primary switchcontrol signal Drive, blanking time signal T_(FMAX), and the Resetsignal. Blanking time signal T_(FMAX) can be used to select the on-setof the modulation switch turn-on signal, as described above inconnection with FIGS. 7A and 7B. It also limits the lower bound of theswitching period.

FIG. 9 is a simplified schematic diagram illustrating a first example ofa quasi-resonant controller that embodies certain aspects of thisinvention. FIG. 10 is a simplified schematic diagram illustrating asecond example of a quasi-resonant controller that embodies certainaspects of this invention. As shown in FIGS. 9 and 10, quasi-resonantcontrollers 900 and 1000 are examples of quasi-resonant controllers thatcan be used as quasi-resonant controller 153 in converter 100 in FIG. 1.As shown in FIG. 9, quasi-resonant controller 900 has a first inputterminal 901 for receiving the modulation switch turn-off signal, asecond input terminal 902 for receiving the valley pulse signalV_(Pulse), and an output terminal 903 for providing a primary switchturn on signal Trigger. Similarly, as shown in FIG. 10, quasi-resonantcontroller 1000 has a first input terminal 1001 for receiving themodulation switch turn-off signal, a second input terminal 1002 forreceiving the valley pulse signal V_(Pulse), and an output terminal 1003for providing a primary switch turn on signal Trigger. Bothquasi-resonant controller 900 and quasi-resonant controller 1000 providea primary switch turn-on signal at a valley point of the resonantwaveform in the discontinuous time.

As shown in FIG. 9, quasi-resonant controller 900 has a D flipflop 910that includes a first input terminal 901 for receiving the modulationswitch off signal Mod_Off, a second input terminal 902 for receiving thevalley pulse signal V_(Pulse), and an output terminal 903 for providingthe primary switch turn on signal Trigger. Waveform Mod illustrates themodulation switch turn-on signal. Waveform Mod_Off indicates that themodulation switch is turned off after the modulation switch has been onfor a duration determined by the jitter control circuit. WaveformV_(Pulse) shows the valley pulse signals. D flipflop 910 produces thetrigger signal Trigger at the rising edge of the first valley pulsesignal V_(Pulse) after the modulation switch is turned off. The primaryswitch control signal Drive is provided by control circuit 150 in FIG. 1in response to the Trigger signal. The turning on of primary switchindicates the beginning of a new switching cycle, and the D flipflop 910receives a global signal Reset to standby for the new cycle.

As shown in FIG. 10, quasi-resonant controller 1000 has two D flipflopsconnected in series. A first D flipflop 1010 includes a first inputterminal 1011 for receiving the modulation switch off signal Mod_Off,and a second input terminal 1012 for receiving the valley pulse signalV_(Pulse). A second D flipflop 1020 has a first input terminal 1021 forreceiving an output signal from the first D flipflop 1010, and a secondinput terminal 1022 for receiving the valley pulse signal. The second Dflipflop 1020 also has an output terminal 1023 for providing the primaryswitch trigger signal Trigger. FIG. 10 also shows waveforms thatillustrate the same signals as those shown in FIG. 9. A differencebetween quasi-resonant controller 1000 and quasi-resonant controller 900is that quasi-resonant controller 1000 includes two D flipflops, whereasquasi-resonant controller 900 has only one D flipflop. As a result, inquasi-resonant controller 1000, the primary switch is turned on at thesecond valley point after the modulation switch is turned off. Thequasi-resonant controller 1000 makes the valley count different from thejitter peak count and the system switching frequency has a wider jitterrange.

As described above, quasi-resonant controller 900 has one D flipflop,and quasi-resonant controller 1000 has two D flipflops connected inseries. More generally, each quasi-resonant controller can have aflipflop chain having one D flipflop or two or more serially-connected Dflipflops. A first one of the one or more D flipflops has a first inputterminal for receiving the modulation switch off signal and a secondinput terminal for receiving the valley pulse signal. A last one of theone or more D flipflops has an output terminal for providing the primaryswitch trigger signal. Each D flipflop other than the first one of theone or more D flipflops has a first input terminal for receiving anoutput signal from a preceding D flipflop and a second input terminalfor receiving the valley pulse signal. Even though the above examplesinclude D flipflops, it is understood that similar functions can beimplemented using other flipflops or latches that have two stable statesand can be used to store state information.

FIG. 11A is a simplified schematic diagram illustrating a quasi-resonant(QR) converter with jitter that embodies certain aspects of thisinvention. FIG. 11B is a waveform diagram illustrating the operation ofthe quasi-resonant (QR) converter with the jitter of FIG. 11B thatembodies certain aspects of this invention. FIG. 11A shows part ofquasi-resonant (QR) converter 1100, which is similar to quasi-resonant(QR) converter 100 of FIG. 1, that illustrates that the modulationcapacitor and the modulation switch are coupled with the primary switchin parallel between an input node of the power switch and a groundterminal. In this case, the power switch Q_(L) is an NMOS transistor andnode 1101 is a drain node of the power switch. The modulation switchQ_(M) is a PMOS transistor, which has a source node 1103 coupled to theground node GND. In this case, the modulation switch Q_(M) is alsoreferred to as a low-side PMOS. FIG. 11B shows that the modulationswitch is turned on at the first peak 1111 of the resonant waveform inthe discontinuous time, and the primary switch is turned on at the thirdvalley 1113 of the resonant waveform in the discontinuous time. Asdescribed above, during the time when the modulation switch is turnedon, the modulation capacitor is connected, and the oscillation period ofthe resonant waveform is T_(QR1). When the modulation switch is turnedoff, the modulation capacitor is disconnected, and the oscillationperiod of the resonant waveform is T_(QR). Accordingly, with atime-varying duration of T_(QR1), a jitter is added to thequasi-resonant converter.

FIG. 12A is a simplified schematic diagram illustrating a quasi-resonant(QR) converter with jitter that embodies certain aspects of thisinvention. FIG. 12B is a waveform diagram illustrating the operation ofthe quasi-resonant (QR) converter with the jitter of FIG. 12A thatembodies certain aspects of this invention. FIG. 12A shows part ofquasi-resonant (QR) converter 1200, which is similar to quasi-resonant(QR) converter 100 of FIG. 1, except that the modulation capacitor andthe modulation switch are coupled with the primary switch in parallelbetween an input node of the power switch and a power terminal. In thiscase, the power switch Q_(L) is an NMOS transistor and node 1201 is adrain node of the power switch. The modulation switch Q_(M) is an NMOStransistor, which has a drain node 1203 coupled to a power node inseries with the modulation capacitor, in this case, V_(in). In thiscase, the modulation switch Q_(M) is also referred to as a high-sideNMOS. The NMOS Q_(M) needs a bootstrap driving circuit to provide acontrol signal for the swinging source voltage. Node SW provides areference ground for the control signal.

The operation of quasi-resonant (QR) converter 1200 in FIG. 12B issimilar to the operation of quasi-resonant (QR) converter 1100 in FIG.11B. As shown in FIG. 12B, the modulation switch is turned on at thefirst peak 1211 of the resonant waveform in the discontinuous time, andthe primary switch is turned on at the third valley 1213 of the resonantwaveform in the discontinuous time. Similar to above, during the timewhen the modulation switch is turned on, the oscillation period of theresonant waveform is T_(QR1). When the modulation switch is turned off,the oscillation period of the resonant waveform is T_(QR). Accordingly,a jitter is added to the quasi-resonant converter.

FIG. 13 is a simplified flowchart that illustrates a method forcontrolling a quasi-resonant (QR) converter that embodies certainaspects of this invention. As shown in FIG. 13, method 1300 includes, at1310, identifying valley points and peak points of a resonant waveformin a sensing signal during a discontinuous time of the converter. Method1300 also includes, at 1320, turning on a modulation switch to add acapacitance in parallel to the power switch at a peak point of theresonant waveform in the sensing signal during the discontinuous time,to vary an oscillation period of the resonant waveform to add afrequency jitter to a switching frequency of the converter. Method 1300also includes, at 1330, turning on a power switch at a valley point of aresonant waveform in a sensing signal after the modulation switch hasbeen turned off. The power switch is coupled to a primary winding of theconverter to control a primary current flow, and the sensing signal isrepresentative of the resonant waveform.

In method 1300, adding a capacitance in parallel to the power switch caninclude turning on a modulation switch that is coupled in series with amodulation capacitor. Method 1300 also includes turning on themodulation switch at a first peak point in the resonant waveform after ablanking time, and turning off the modulation switch after a time periodbased on a time-varying function to vary a turn-on time of themodulation switch. The method can also include turning on the powerswitch at a valley point of the resonant waveform after a preset offtime of the modulation switch.

FIG. 14 is a simplified schematic diagram illustrating a power converterwith jitter that embodies certain aspects of this invention. FIG. 14shows part of a power converter 1400, which is similar to flybackconverter 100 of FIG. 1, including the inductance L_(m) of the primarywinding and auxiliary winding AUX providing VCC. In this example, amodulation capacitor C_(SN) is shunted with parasitic capacitor C_(OSS)to change the resonant period and modulation switch Q_(M) is in serieswith C_(SN) to control the shunted time t_(jitter).

FIG. 15 illustrates the waveforms for the power converter of FIG. 14that embodies certain aspects of this invention. FIG. 15 plots the Drivesignal that turns on the power switch Q_(L), the Mod signal that turnson the modulation switch Q_(M), and the drain-source voltage V_(DS) ofthe power switch Q_(L). As shown in FIG. 15, the switching period of theflyback system can be varied by different t_(jitter) which changes thequasi-resonant oscillation period from T_(QR) to T_(QR1), where, asdescribed above in connection with FIGS. 5A and 5B, T_(QR)=2π√{squareroot over (L_(m)C_(OSS))} is the oscillation period of resonant waveformwith modulation switch Q_(M) turned off, and T_(QR1)=2π√{square rootover (L_(m)(C_(OSS)+C_(sn)))} is the oscillation period of resonantwaveform with modulation switch Q_(M) turned on.

By linearly changing the t_(jitter), the switching frequency of theconverter can be spread out in a frequency range as shown in FIG. 16,similar to that described above in connection with FIGS. 4A-4D. Thebenefits of spectrum separating for electro-magnetic interference (EMI)and quasi-resonant control for C_(OSS) switching loss can be achieved atthe same time. However, the inventor has recognized that the switchingloss of the power converter can be further improved. For example, theC_(OSS) switching loss can still be large with high input voltageV_(IN).

FIG. 17 plots the Drive signal that turns on the power switch Q_(L), theMod signal that turns on the modulation switch Q_(M), the drain-sourcevoltage V_(DS) of the power switch Q_(L), and the current I_(m) in theprimary winding L_(m). As shown in FIG. 17, the C_(SN) changes theresonant period, but keeps the resonant amplitude the same at theboundary of frequency modulation region. As a result, the switching lossis still similar to the traditional quasi-resonant control, and theflyback system can have lower efficiency with high input AC voltage.

This invention further teaches a technique to reduce the switching lossof C_(OSS) associated with the power switch for the power converter. Inan example, in order to reduce the switching loss of the power switchC_(OSS), the inductor current I_(m) flowing into input voltage sourceV_(IN) is kept in inductor L_(m) at the end of the modulation switchQ_(M) modulation time.

FIG. 18 shows an example of generating the negative inductor currentI_(M). FIG. 18 plots the Drive signal that turns on the power switchQ_(L), the Mod signal that turns on the modulation switch Q_(M), thedrain-source voltage V_(DS) of the power switch Q_(L), and the currentI_(m) in the primary winding L_(m). In an example, a large C_(SN) isselected to have T_(QR1)>>t_(jitter_MAX), and the modulation time ofQ_(M) includes a dithering t_(jitter)(t) variant with time and an offsetT(V_(IN)) controlled by input voltage V_(IN). In other words, thefrequency jitter includes a first portion that varies with the inputvoltage of the power converter and a second portion that is atime-varying function. With a high V_(IN), the large T(V_(IN)) causesthe L_(m) to have a large negative I_(m) as soon as the Q_(M) is turnedoff. This initial current increases the amplitude of the followingquasi-resonant waveform and reduces the valley switching voltage onC_(OSS).

FIGS. 19A-19C plot the Mod signal that turns on the modulation switchQ_(M), the drain-source voltage V_(DS) of the power switch Q_(L), andthe current I_(m) in the primary winding L_(m) at different inputvoltages V_(IN). FIG. 19A shows the waveforms at a high V_(IN), FIG. 19Bshows the waveforms at a medium V_(IN), and FIG. 19C shows the waveformsat a low V_(IN). With smaller input voltage V_(IN), the smallerT(V_(IN)) can induce less I_(m) and less resonant amplitude to adapt toless Q_(L)-V_(DS) on capacitance C_(OSS).

FIG. 20 shows the example of generating the negative I_(m) at lowV_(IN). Similar to FIG. 18, FIG. 20 plots the Drive signal that turns onthe power switch Q_(L), the Mod signal that turns on the modulationswitch Q_(M), the drain-source voltage V_(DS) of the power switch Q_(L),and the current I_(m) in the primary winding L_(m). However, in FIG. 20,the V_(IN) is relatively low, and T(V_(IN)) is small so the modulationswitch Q_(M) is turned off earlier. It induces less negative I_(m) andless quasi-resonant amplitude subsequently, which is suitable for thelow V_(IN) since the resonant waveform already have low V_(DS) drop atthe valley point.

FIG. 21 is a simplified schematic diagram of a power converter thatembodies certain aspects of this invention. Power converter 2100includes many components similar to those in converter 100 of FIG. 1, asdescribed in detailed below. However, in power converter 2100, an inputvoltage magnitude detector 2160 is added to provide a signal f(V_(IN))to the jitter controller 2154 to provide a frequency jitter thatincludes a first portion that varies with the input voltage of the powerconverter and a second portion that is a time-varying function. Theswitching loss of the power transistor is reduced and power efficiencycan be improved.

Power converter 2100 includes a transformer 2102, which includes aprimary winding 2141 for coupling to an input voltage V_(IN) and asecondary winding 2142 for providing the output voltage V_(OUT) througha rectifying diode 2120 and a capacitor 2119. A current in therectifying diode is I_(D). Transformer 2102 also has an auxiliarywinding 2143 for providing a sensing signal DMEG monitoring a resonantwaveform of the converter during a discontinuous time. Auxiliary winding2143 also provides a voltage V_(CC), which can be used as a power supplyfor the support circuitry, such as a control circuit. In FIG. 21, Vpdenotes the voltage at the primary winding, Vs denotes the voltage atthe secondary winding, and Va denotes the voltage at the auxiliarywinding. In FIG. 21, L_(m) represents the inductance of the primarywinding, and N represents the ratio of coil turns in the primary windingto the secondary winding. C_(oss) represents the capacitance associatedwith primary switch 2101, including the capacitance from the body diode.Vds represents the drain-source voltage across the primary switch.

Converter 2100 includes a control circuit 2150. Converter 2100 alsoincludes a power switch 2101 (also designated as Q_(L)) coupled toprimary winding 2141 and control circuit 2150 for receiving a controlsignal to turn on and off power switch 2101 to control the primarycurrent through primary winding 2141 in order to regulate output voltageV_(OUT). In FIG. 21, power switch 2101 is shown as a MOSFET powertransistor. In the embodiment of FIG. 21, control circuit 2150 canreceive its operating power from V_(cc) provided by the auxiliarywinding. Control circuit 2150 can also receive sensing signal DMEG fromauxiliary winding 2143. Control circuit 2150 can also receive a currentsense signal representative of the primary current through a currentsense resistor with a resistance (as shown in FIG. 1, but not shown inFIG. 21). Control circuit 2150 provides a control signal Drive tocontrol power switch 2101. Control circuit 2150 turns on the powerswitch 2101 based on information provided by the sensing signal DMEG andturns off the power 2101 based on information provided by the currentsense signal Vcs.

As shown in FIG. 21, converter 2100 also has a modulation capacitor 2130(also designated as C_(SN)) and a modulation switch 2131 (alsodesignated as Q_(M)) coupled in parallel to power switch 2101. In thisarrangement, modulation switch 2131 is coupled in series with capacitor2130. Control circuit 2150 includes a valley detector 2151 for detectingvalley points in the resonant waveform in the sensing signal DMEG duringthe discontinuous time of converter 2100. Control circuit 2150 alsoincludes a peak detector 2152 for detecting peak points in the resonantwaveform in the sensing signal during the discontinuous time. Controlcircuit 2150 also includes a power controller 2153 for turning on thepower switch 2101 at a valley point of a resonant waveform in thesensing signal during a discontinuous time. Control circuit 2150 furtherincludes a jitter controller 2154 for turning on the modulation switch2131 at a peak point of the resonant waveform in the sensing signalduring the discontinuous time. Control circuit 2150 also includes anoscillator OSC 2155, which provides a T_(FMAX) signal to jittercontroller 2154. In addition to the T_(FMAX) signal, jitter controller2154 also receives a Ppulse signal from peak detector 2152, and providesa Mod_Off signal to power controller 2153. Power controller 2153 alsoreceives a Vpulse signal from valley detector 2151. Control circuit 2150also includes a flipflop 2156 that outputs the Drive signal.

As explained in detail below, jitter controller 2154 varies a turn-ontime of the modulation switch 2131 to add a frequency jitter to theswitching frequency of the converter. The waveform of Vds is mirrored tothe auxiliary winding and detected by a peak detector and a valleydetector. The jitter controller determines the target peak count to turnon Q_(M) and linearly modulates the on time of Q_(M). After Q_(M) isturned off, the power controller is enabled by the jitter controller towait for proper valley count and turn on Q_(L) at the valley of Vds.

Power converter 2100 also includes an input voltage magnitude detector2160 to provide a signal f(V_(IN)) to the jitter controller 2154 toprovide a frequency jitter that includes a first portion that varieswith the input voltage of the power converter and a second portion thatis a time-varying function. In the example of FIG. 21, the input voltagemagnitude detector 2160 includes a sample-and-hold block (V_(IN) S/H) tosense the V_(IN) information through the DMEG node during the on-time ofQ_(L). The sampled f(V_(IN)) is used to add a variable of Q_(M)modulation time which is T(V_(IN))+t_(jitter)(t). In an example, thecontroller turns on the Q_(M) at a peak of V_(ds) and turns off Q_(M)according to the Q_(M) modulation time. After the Q_(M) is turned off,the V_(ds) falls rapidly due to the increasing amplitude ofquasi-resonant waveform. Thus the valley detector sends a signal to turnon Q_(L) and a new switching cycle begins.

FIG. 22 shows schematic and waveform diagrams illustrating thestructures and functions of a valley detector and a peak detector in thecontrol circuit in a converter that embodies certain aspects of thisinvention. In FIG. 22, a valley detector 2210 includes a firstcomparator 2211 for comparing a sensing signal DMEG and a valleyreference voltage Vref_V. A peak detector 2220 includes a secondcomparator 2222 for comparing a sensing signal DMEG and a peak referencevoltage Vref_P. Waveform 2230 illustrates the sensing signal DMEG, withthe valley reference voltage Vref_V and peak reference voltage Vref_P.Waveform 2240 illustrates the output waveform of the second comparator2222, showing a pulse signal P_(Pulse) where the sensing signal DMEG isgreater than peak reference voltage Vref_P. Waveform 2250 illustratesthe output waveform of the first comparator 2211, showing a pulse signalV_(Pulse) where the sensing signal DMEG is less than valley referencevoltage Vref_V. This example detects peaks and valleys of V_(ds) throughthe DMEG voltage. The rising edge of P_(Pulse) and V_(Pulse) are sent toturn on Q_(M) at peak of V_(ds) and turn on Q_(L) at valley of V_(ds).

FIG. 23 shows a schematic for an input voltage magnitude detector 2300,which can be used as input voltage magnitude detector 2160 in controlcircuit 2150 in FIG. 21. As shown in FIG. 23, input voltage magnitudedetector 2300 includes an inverting amplifier 2310 and a sample-and-holdblock 2320 to sense the V_(IN) information through the DMEG node duringthe on-time of Q_(L). During the on-time of Q_(L), the V_(IN) is appliedon the primary side of transformer to store energy into inductor L_(m),and the auxiliary winding induces a negative voltage proportional toV_(IN) according to the turn ratio. With the inverting amplifier, theV_(IN) information on DMEG node is converted into a signal which is afunction of V_(IN), and a sample and hold circuit is applied to keep thesignal measured during the Q_(L) on time. The sampled signal f(V_(IN))is used to add a variable of Q_(M) modulation time which isT(V_(IN))+t_(jitter)(t). Inverting amplifier 2310 is coupled to areference signal V_(OS). A switch in the ample-and-hold block 2320 iscontrolled by the Drive signal. FIG. 23 also plots the Drive signal thatturns on the power switch Q_(L), the DMEG signal, which has a negativeamplitude proportional to V_(IN) during the Q_(L) on time, and theoutput f(V_(IN)) of the input voltage magnitude detector 2300.

FIG. 24 is a simplified schematic diagram illustrating a jittercontroller, and FIG. 25 illustrates a waveform diagram that describesthe operation of the jitter controller that embodies certain aspects ofthis invention. As shown in FIGS. 24 and 25, jitter controller 2400 isan example of a jitter controller that can be used as jitter controller2154 in converter 2100 in FIG. 21. Jitter controller 2400 includes two Dflipflops. A first D flipflop 2410 has a first input terminal 2411 forreceiving a blanking time signal T_(FMAX), a second input terminal 2412for receiving the peak pulse signal P_(Pulse), a first output terminal2413 for providing a modulation-on signal Mod_On, and a second outputterminal 2414 for providing a complement of the modulation on signal.

Jitter controller 2400 also includes a jitter duration circuit 2420having a first ramp signal circuit 2421 with a current source 2422 andcapacitor 2423 that starts charging the capacitor upon receiving thecomplement of the modulation on signal, from the second output terminal2414 of the first D flipflop 2410, to produce a first ramp signalV_(RAMP1). Jitter duration circuit 2420 also has a first comparator 2425for comparing the first ramp signal V_(RAMP1) and an input voltagereference voltage f(V_(IN)) and providing a jitter offset signal 2427.

Jitter duration circuit 2420 also has a second ramp signal circuit 2451with a current source 2452 and capacitor 2453 that starts charging thecapacitor upon receiving jitter offset signal 2427 through an inverter2428 to produce a second ramp signal V_(RAMP2). Jitter duration circuit2420 also has a comparator 2455 for comparing the second ramp signalV_(RAMP2) and a time-varying jitter reference voltage V_(jitter)(t) andproviding a jitter stop signal 2457.

In FIG. 24, a second D flipflop 2430 has a first input terminal 2431 forreceiving logic high signal Logic H, a second input terminal 2432 forreceiving the modulation on signal Mod_On from the first D flipflop2410, a reset terminal 2435 for receiving the jitter stop signal 2457, afirst output terminal 2433 for providing a modulation switch turn-onsignal Mod, and a second output terminal 2434 for providing a complementof the modulation switch turn-on signal. Jitter controller 2400 also hasan AND circuit 2440 for receiving the modulation on signal Mod_On andthe complement of the modulation switch turn-on signal Mod and providinga modulation switch turn-off signal Mod_Off.

FIG. 25 shows example waveforms of the jitter controller 2400, alsoreferred to as the modulation time controller. FIG. 25 plots the Drive,T_(FMAX), P_(Pulse), V_(RAMP1), V_(RAMP2), Mod, Mod_Off, and Resetsignals. As shown in FIG. 25, the beginning of the modulation time istriggered by the first P_(Pulse) signal after a blanking time signalT_(FMAX) goes high. The T_(FMAX) signal in FIG. 25 is generated by atimer shown in FIG. 26 to limit the maximum value of the systemswitching frequency, and the P_(Pulse) is signal from the peak detectorto turn on Q_(M) with less switching loss. As described above inconnection with FIG. 24, the length of modulation time MOD is determinedby two timers in series. The first timer generates a delay T(V_(IN))according to the signal f(V_(IN)), and the second timer generatesanother delay t_(jitter)(t) according to a time variant signalV_(jitter)(t). The summation of the two delays determines a signal Modto drive Q_(M) and achieves the modulation time with time variantdithering and V_(IN) controlled offset. In some embodiments, thetime-varying signal can be provided by an oscillator, for example.

After the Q_(M) turn off, the modulation time controller enables aMod_Off flag. Since the offset of modulation time of Q_(M) increases theresonant amplitude of V_(ds), the Q_(L) should be turned on at the firstvalley after the modulation time to have lower switching loss.

FIG. 26 shows a simplified schematic diagram illustrating a blankingtime generation circuit and a waveform diagram that describes theoperation of timer circuit 2600 that can be used as a blanking timegeneration circuit that embodies certain aspects of this invention.Timer circuit 2600 includes a ramp signal circuit 2610 with a currentsource and capacitor that starts charging the capacitor upon receiving aReset signal to produce a ramp signal. As shown in FIG. 26, the Resetsignal is a pulse signal produced by a rising edge one-shot circuit 2620at the rising edge of the primary switch Q_(L) turn-on signal. Blankingtime generation circuit 2600 also has a comparator 2630 for comparingthe ramp signal and a reference voltage V_(FMAX) and providing blankingtime signal T_(FMAX). FIG. 26 also shows the waveforms for the primaryswitch control signal Drive, blanking time signal T_(FMAX), and theReset signal. Blanking time signal T_(FMAX) can be used to select theon-set of the modulation switch turn-on signal, as described above inconnection with FIGS. 24 and 25. It also limits the lower bound of theswitching period.

FIG. 27 is a simplified schematic diagram illustrating a first exampleof a power controller that embodies certain aspects of this invention.As shown in FIG. 27, power controller 2700 is an example of powercontroller that can be used as power controller 2153 in converter 2100in FIG. 21. As shown in FIG. 27, power controller 2700 has a first inputterminal 2701 for receiving the modulation switch turn-off signalMod_Off, a second input terminal 2702 for receiving the valley pulsesignal V_(Pulse), and an output terminal 2703 for providing a primaryswitch turn on signal Trigger. Power controller 2700 provides a primaryswitch turn-on signal at a valley point of the resonant waveform in thediscontinuous time.

As shown in FIG. 27, power controller 2700 has a D flipflop 2710 thatincludes a first input terminal 2701 for receiving the modulation switchoff signal Mod_Off, a second input terminal 2702 for receiving thevalley pulse signal V_(Pulse), and an output terminal 2703 for providingthe primary switch turn on signal Trigger. Waveform Mod illustrates themodulation switch turn-on signal. Waveform Mod_Off indicates that themodulation switch is turned off after the modulation switch has been onfor a duration determined by the jitter control circuit. WaveformV_(Pulse) shows the valley pulse signals. D flipflop 2710 produces thetrigger signal Trigger at the rising edge of the first valley pulsesignal V_(Pulse) after the modulation switch is turned off. The primaryswitch control signal Drive is provided by control circuit 2150 in FIG.21 in response to the Trigger signal. Turning on the primary switchindicates the beginning of a new switching cycle, and the D flipflop2710 receives a global signal Reset to standby for the new cycle. Thepower controller sends the trigger signal with the first V_(Pulse) afterthe Mod_Off flag is enabled. The trigger signal turns on Q_(L) andbegins the next switching cycle of system.

FIG. 28 is a simplified flowchart that illustrates a method forcontrolling a power converter that embodies certain aspects of thisinvention. As shown in FIG. 28, method 2800 includes, at 2810, turningon a power switch in response to a resonant waveform in a sensing signalduring a discontinuous time of the power converter. Method 2800 alsoincludes, at 2820, adding a capacitance in parallel to the power switchduring the discontinuous time, to vary an oscillation period of theresonant waveform to add a frequency jitter to a switching frequency ofthe power converter.

Examples of power converters implementing method 2800 are describedabove in connection with FIGS. 14-27. In some embodiments, the frequencyjitter can include a first portion that varies with an input voltage ofthe power converter and a second portion that is a time-varyingfunction. In method 2800, adding a capacitance in parallel to the powerswitch can include turning on a modulation switch that is coupled inseries with a modulation capacitor. Method 2800 can also include turningon a power switch in response to a resonant waveform in a sensing signalduring a discontinuous time of the power converter, wherein the powerswitch is coupled to a primary winding of the power converter to controla primary current flow. The method can also include adding a capacitancein parallel to the power switch during the discontinuous time, to varyan oscillation period of the resonant waveform to add a frequency jitterto a switching frequency of the power converter. The method can add acapacitance in parallel to the power switch by turning on a modulationswitch that is coupled in series with a capacitor, wherein the capacitorand the modulation switch are coupled in parallel with the power switch.The frequency jitter can include a first portion that varies with aninput voltage of the power converter and a second portion that is atime-varying function.

FIG. 29 is a simplified waveform diagram illustrating signals for aflyback power controller that embodies certain aspects of thisinvention. FIG. 29 illustrates part of a power controller 2900, which issimilar to flyback converter 2100 of FIG. 21. FIG. 29 illustratessimilar components as those shown in FIG. 21. FIG. 29 also illustratescontrol signals and waveforms of a modulation circuit in the powercontroller, similar to those shown in FIG. 18. Therefore, detaileddescriptions of the common components and signals are not repeated here.The modulation circuit is implemented to reduce switching loss offlyback system. During the discontinuous time of the system, amodulation switch Q_(M) is turned on to connect a modulation capacitorC_(SN) in parallel with the C_(OSS) at the peak of the resonantwaveform. The increasing total capacitance can enlarge the resonantcurrent on the magnetizing inductor L_(m). By turning off Q_(M) at theproper time with larger negative i_(m)(t) as shown in the plot ofI_(flux) of L_(m) in FIG. 29, the following resonant waveform can havelarger voltage amplitude and the control circuit can turn on the powerswitch Q_(L) with less V_(DS) drop, which means less switching loss.

However, this control method illustrated in FIG. 29 can have a drawbackon transformer design. In order to precisely control the negativecurrent on L_(m), the control circuit needs to accurately detect thepeak of a resonant waveform for turning on the Q_(M). As a result, thetransformer must be operated under discontinuous conduction mode (DCM).

FIG. 30 is a simplified waveform diagram illustrating signals forflyback controller 2900 of FIG. 29 operated under discontinuousconduction mode (DCM) that embodies certain aspects of this invention.FIG. 30 shows the waveforms of the signals under full load and low inputvoltage. For example, t_(ON) is the on time of power switch Q_(L), andt_(OFF) is the off time of power switch Q_(L). The off time t_(OFF) caninclude a demagnetization-time t_(DEMAG) and a discontinuous timet_(DIS). During the demagnetization-time t_(DEMAG), the secondarywinding discharges, and during the discontinuous time t_(DIS), nocurrent flows, but a resonant waveform can be detected. Thedemagnetization-time t_(DEMAG) can be expressed as

$t_{DEMAG} = {\frac{V_{IN}}{{NV}_{OUT}}{t_{ON}.}}$

In this example, at the beginning of a switching cycle, the low inputvoltage causes a long power switch Q_(L) on-time t_(ON). Further, at theend of the switching cycle, the control circuit needs a discontinuoustime t_(DIS) to detect the peak of the quasi-resonant (QR) waveform andturn on modulation switch Q_(M). As shown in FIG. 30, the peak currentcan be expressed as

$I_{{peak}\;\_\;{Mod}} = {\frac{2}{N}I_{OUT} \times {\frac{t_{ON} + {\frac{V_{IN}}{{NV}_{OUT}}t_{ON}} + t_{DIS}}{\frac{V_{IN}}{{NV}_{OUT}}t_{ON}}.}}$

It can be seen that the t_(ON) and t_(DIS) reduce the duty of t_(DEMAG),and the system needs larger peak current I_(peak_Mod) to deliver theoutput current I_(OUT). The higher peak current means that thetransformer needs lower equivalent series resistance (ESR) and highermaximum flux density (B_(MAX)). Both of these requirements increase thesize and cost of transformer.

An aspect of this invention teaches to change the operating mode of theflyback system according to the input voltage. The efficiency of aflyback system is determined by different factors depending on the inputvoltage. The efficiency of a flyback system is often dominated byswitching loss under high input voltage, and can be dominated byconduction loss under low input voltage. By changing the operating mode,the flyback system can benefit from lower peak current under low inputvoltage and can also benefit from drain-source voltage V_(DS) reductionunder high input voltage.

An aspect of this invention teaches a control method to change theturn-on timing of modulation switch Q_(M) in the flyback systemaccording to the input voltage. When the input voltage is low, the Q_(M)is turned on in the demagnetizing time. During the Q_(M) on-time, theleakage inductor energy, which is stored into C_(SN) when power switchQ_(L) is turned off, is released to V_(OUT) in parallel with theI_(flux) through secondary side diode. Since the Q_(M) does not have tobe turned on again at the end of switching cycle, duty of t_(DEMAG)increases with the reduced this and the peak current in each switchingcycle can be reduced.

FIG. 31 is a simplified waveform diagram illustrating signals forflyback controller 2900 of FIG. 29 operated under quasi-resonant (QR)that embodies certain aspects of this invention. FIG. 31 shows thewaveforms of the QR mode flyback under low input voltage. The powerswitch Q_(L) is turned on at the first valley of quasi-resonantwaveform. It reduces the t_(DIS) to ½T_(QR) and increases the turn-onV_(DS) of Q_(L) to V_(IN)−NV_(OUT). Since the VDS is still small withlow V_(IN), this trade-off can slightly improve total efficiency andease the requirement of transformer with less maximum flux density.

FIG. 32 is a simplified waveform diagram illustrating signals forflyback controller 2900 of FIG. 29 operated under continuous conductionmode (CCM) that embodies certain aspects of this invention. FIG. 32shows the waveforms of the CCM flyback under low input voltage. SinceQ_(L) is turned on when the I_(flux) is still flowing to V_(OUT), thepeak current can be further reduced because of the trapezoidaldemagnetizing current. The greatly reduced maximum flux density can savethe cost and size of the transformer, but the switching loss of Q_(L)increases because of the increasing turn-on V_(DS) V_(IN)+NV_(OUT).However, the low input voltage eases the drawback, and the low peakcurrent may restore some efficiency with smaller conduction loss.

FIG. 33 is a simplified waveform diagram illustrating signals forflyback controller 2900 of FIG. 29 operated in high input voltage modethat embodies certain aspects of this invention. FIG. 33 shows thesystem waveforms under high input voltage. When the input voltage ishigh, the Q_(M) is turned on at the end of t_(DIS) to induce negativei_(m)(−) for reducing Q_(L) switching loss. Since the t_(ON) of Q_(L) isgreatly reduced with high input voltage, the saved time can cover therequirement of Q_(M) control. The flyback system may have similar peakcurrent compared with low input voltage. It means that the controlmethod can improve switching loss with the same transformer design.

FIG. 34 is a simplified schematic diagram of a power converter thatembodies certain aspects of this invention. Power converter 3400includes many components similar to those in converter 2100 in FIG. 21and converter 100 of FIG. 1. Therefore, detailed descriptions of thecommon components and operations are omitted here. It is noted that, inpower converter 3400, controller circuit 3450 includes a high/low inputvoltage mode selector 3410, which provides a signal “flag” to modulationcontroller 3454 for changing the operating mode of the flyback systembetween low input voltage mode and high input voltage mode depending onthe input voltage. As a result, the switching loss or conduction loss ofthe power transistor is reduced and power efficiency can be improved.

Mode selector 3410 in control circuit 3450 detects the input voltage ofthe system and sends a “flag” signal 3411 that determines the operatingmode. In a high input voltage mode, the modulation switch Q_(M) isturned on during a discontinuous time to generate the negative currenti_(m)(−) on L_(m) for reducing switching loss of Q_(L). In contrast, ina low input voltage mode, the modulation switch Q_(M) is turned onduring demagnetization-time to release the leakage energy stored inC_(SN) to the output. Control circuit 3450 also includes a valleydetector 3451, a peak detector 3452, a power controller 3453, amodulation controller 3454 (labeled as Q_(M) controller), an oscillator3455, and a flipflop 3456. The functions of these components aredescribed below.

FIG. 35 is a simplified schematic diagram for a mode selector thatembodies certain aspects of this invention. As shown in FIG. 35, modeselector 3500 is an example of a mode selector that can be used as modeselector 3410 in power converter 3400 of FIG. 34. As shown, modeselector 3500 includes a voltage divider 3510 for sensing the inputvoltage V_(IN) and a comparator 3520 for outputting a signal 3530,labeled “flag” in FIG. 35. The signal 3530, labeled “flag,” indicateswhether the input voltage V_(IN) is high or low, and is used forchanging the operating mode of the controller. As shown in FIG. 35,f(V_(IN)) is a sampled value of input voltage V_(IN) and V_(ref) is areference voltage. The output of comparator 3520 sets the signal “flag”to 1, if V_(IN) is greater than V_(ref), and sets the signal “flag” to0, if V_(IN) is less than V_(ref). As an example, the power convertercan operate with an input voltage of either 110V or 220V. In this case,the parameters in the voltage divider and the reference voltage can beselected such that the power converter operates in the low input voltagemode if V_(IN) is less than, e.g., 150V, and in the high input voltagemode, otherwise.

FIG. 36 is a simplified schematic diagram for an alternative modeselector that embodies certain aspects of this invention. As shown inFIG. 36, mode selector 3600 is another example of a mode selector thatcan be used as mode selector 3410 in power converter 3400 of FIG. 34. Asshown in FIG. 36, mode selector 3600 obtains the information of inputvoltage through the auxiliary winding, for example, the DMEG signal inpower converter 3400 in FIG. 34. Mode selector 3600 includes aninverting amplifier 3610 to sense the signal on DMEG during the on-timeof the power switch Q_(L), as shown by the Drive signal. Mode selector3600 also includes a sample and hold circuit 3620 to provide an outputsignal f(V_(IN)), which is an input to a comparator 3630 to generate the“flag” signal for determining the operating mode of the converter.

FIG. 37 is a simplified schematic diagram for an example of the Q_(M)controller in power converter 3400 of FIG. 34 that embodies certainaspects of this invention. As shown in FIG. 37, modulation controller3700, also referred to as Q_(M) controller, includes an inverter 3710, adelay circuit 3720, a one-shot circuit 3730, multiplexers 3741 and 3742,a first D-flipflop 3750, a second D-flipflop 3760, a Q_(M) on-timecalculator 3770, and an AND circuit 3780. The Q_(M) controller 3700provides a Mod signal to drive the modulation switch Q_(M), and a QR_ENsignal to enable the power controller block 3453 in flyback controller3400 in FIG. 34. Two multiplexers are controlled by the flag signal toselect the output signals Mod and QR_EN for different operating modes ofthe flyback system.

FIG. 38 shows a portion of modulation controller 3700 of FIG. 37 andsignal waveforms for operation in low input voltage mode that embodiescertain aspects of this invention. Modulation controller 3700 (alsoreferred to as Q_(M) controller) is set in the low input voltage mode,when the flag signal is 0. In low input voltage mode, modulation switchQ_(M) is turned on during the demagnetization-time to release theleakage inductance energy stored in C_(SN) to the output. FIG. 38 showsa portion of modulation controller 3700, labeled circuit block 3800, forturning on and off the modulation switch Q_(M) in low input voltagemode. As shown in FIG. 38, circuit block 3800 includes inverter 3710,delay circuit 3720, one-shot circuit 3730, and multiplexers 3741 and3742 in modulation controller 3700 in FIG. 37. The modulation switchQ_(M) is turned on by the Mod signal right after the power switch Q_(L)is turned off by the Drive signal and a small dead-time t_(dead). Asmall on-time of Q_(M) is generated by the one-shot circuit 3730. Sincethe modulation switch Q_(M) is turned on and turned off quickly afterthe beginning of demagnetization-time, the on-time duration of Q_(M) isin the demagnetization-time, and the Q_(M) is turned off before themaximum frequency period T_(FMAX). In this example, in the low inputvoltage mode, the modulation controller is configured to turn on themodulation switch after a pre-set delay time after the power switch isturned off in every switching cycle of the power converter. t_(dead) isselected to ensure Q_(M) is turned on in the demagnetization-time. Forexample, in an application with switching frequencies in the 60-70 kHz,the discharge time or demagnetization-time is 2-20 μsec, and the delaytime can be 200-500 nsec. In applications with higher switchingfrequencies, the delay time can be adjusted accordingly.

FIG. 39 shows another portion of Q_(M) controller 3700 of FIG. 37 andsignal waveforms for operation in high input voltage mode that embodiescertain aspects of this invention. Modulation controller 3700 (alsoreferred to as Q_(M) controller) is set in the high input voltage mode,when the flag signal is 1. In the high input voltage mode, modulationswitch Q_(M) is turned on during the continuous time to release theleakage inductance energy stored in C_(SN) to the output. FIG. 39 showsa portion of modulation controller 3700, labeled circuit block 3900, forturning on and off the modulation switch Q_(M) in high input voltagemode. As shown in FIG. 39, circuit block 3900 is similar to jittercontroller 2400 illustrated in FIG. 24, but without the time-varyingcomponents in the on-time calculator intended for introducing thejitters. As shown, circuit block 3900 includes the first D-flipflop3750, the second D-flipflop 3760, Q_(M) on-time calculator 3770, ANDcircuit 3780, and multiplexers 3741 and 3742 in modulation controller3700 in FIG. 37. Their functions are similar to the correspondingcomponents in FIG. 24.

FIG. 39 also shows the waveforms of the Q_(M) controller in high inputvoltage mode when the flag signal is 1. In high input voltage mode,Q_(M) is turned on during the discontinuous time to induce a negativecurrent on L_(m) before the power switch Q_(L) is turned on. In thisembodiment, Q_(M) is turned on at the first peak of the resonantwaveform after the maximum frequency period T_(FMAX), and is turned offby a Q_(M) on-time calculator. After Q_(M) is turned off, the QR_ENsignal is set high to enable the power controller block, whichdetermines the turn on timing of the power switch Q_(L).

FIG. 40A is a simplified block diagram of an alternative modulationcontroller for a multi-mode flyback system, and FIG. 40B illustratescorresponding signal waveforms for operation in a low input voltage modethat embodies certain aspects of this invention. Modulation controller4000 in FIG. 40 is similar to modulation controller 3700 in FIG. 37. Onedifference is that modulation controller 4000 includes a low-frequencyenable signal to block most of the Mod driving signal in low inputvoltage mode. Since the function of Q_(M) is to release the leakageenergy stored in C_(SN) to output, in some cases, Q_(M) does not need tobe turned on in each switching cycle. In this embodiment, the Q_(M) isturned on in the first switching cycle after the rising edge of a timingsignal T_(Recovery). The timing signal can be selected to extend throughmultiple switching cycles of the power converter. Therefore, themodulation controller is configured to turn on the modulation switchafter the pre-set delay time after the power switch is turned off oncein multiple switching cycles of the power converter. During the otherswitching cycles, Q_(M) stays open to save switching loss and theleakage energy is stacked into C_(SN) through the body diode of Q_(M).This function is implemented with a D flipflop 4010 with an input Logichigh and the timing signal T_(Recovery). FIG. 40B shows various timingsignals for modulation controller 4000.

FIG. 41 illustrates a simplified block diagram of a power controller andits operating waveforms in different operating mode that embodiescertain aspects of this invention. As shown in FIG. 41, power controller4100 is an example of a power controller that can be used as powercontroller 3453 in power converter 3400 in FIG. 34. Many components andsignals in FIG. 41 are similar to those in the examples described above.After the QR_EN is set to high, the power controller generates durationT_(window) to detect the V_(Pulse) signal. If the system is in a highinput voltage mode or low input voltage mode with light load, the powercontroller will send the Trigger signal to turn on Q_(L) at the risingedge of the first V_(Pulse) signal in T_(window). If the system is inlow input voltage mode with a heavy load, there will be no V_(Pulse)signal and the power controller will send the Trigger signal at the endof T_(window) to turn on Q_(L). Thus, the flyback system will beoperated in continuous conduction mode (CCM).

FIG. 42 illustrates a simplified block diagram of another example of apower controller and its operating waveforms in different operating modethat embodies certain aspects of this invention. As shown in FIG. 42,power controller 4200 is another example of a power controller that canbe used as power controller 3453 in power converter 3400 in FIG. 34.With this power controller, the flyback system will not enter continuousconduction mode (CCM). In low input voltage mode, the power controllerkeeps waiting until the first quasi-resonant (QR) valley arrives andsends the Trigger signal to turn on Q_(L). In High Input Voltage mode,the power controller will turn on the Q_(L) after the modulationswitched Q_(M) inducing proper i_(m)(−).

FIG. 43 illustrates a simplified block diagram of an oscillator blockthat embodies certain aspects of this invention. As shown in FIG. 43,oscillator 4300 is an example of an oscillator that can be used asoscillator 3455 in power converter 3400 in FIG. 34. The oscillator block4300 generates a global signal Reset to inform the beginning of a newswitching cycle, and generates a blanking time signal T_(FMAX) for theQ_(M) controller. Since the switching cycle is ended after the operationof Q_(M) controller and power controller, the T_(FMAX) limits the lowerbound of the system switching period.

FIG. 44 illustrates a simplified block diagram of the peak detector andvalley detector and the associated waveforms that embodies certainaspects of this invention. As shown in FIG. 44, circuit 4400 is anexample of a peak detector and valley detector that can be used as peakdetector 3452 and valley detector 3451 in power converter 3400 in FIG.34. FIG. 44 shows an embodiment which detects peaks and valleys ofV_(DS) through DMEG voltage during t_(DIS). The rising edges ofP_(Pulse) and V_(Pulse) are sent to turn on Q_(M) at peak of V_(DS) andturn on QL at valley of V_(DS).

FIG. 45 illustrates a simplified block diagram of an embodiment of aQ_(M) on-time calculator and the associated waveforms that embodiescertain aspects of this invention. As shown in FIG. 45, circuit 4500 isan example of a Q_(M) on-time calculator that can be used as the Q_(M)on-time calculator in modulation converter 3700 in FIG. 37. Q_(M)on-time calculator 4500 determines the on-time of Q_(M) in high inputvoltage mode for specific purpose. With this embodiment, the on-time ofQ_(M) is increasing with the input voltage V_(IN), which can inducelarger negative i_(m)(−) corresponding to larger V_(DS) voltage.

FIG. 46 illustrates a simplified block diagram of another embodiment ofa Q_(M) on-time calculator and the associated waveforms that embodiescertain aspects of this invention. As shown in FIG. 46, circuit 4600 isan example of another Q_(M) on-time calculator that can be used as theQ_(M) on-time calculator in modulation converter 3700 in FIG. 37. InQ_(M) on-time calculator 4600, the on-time of Q_(M) includes an offsetperiod determined by V_(IN) and a time varying period. This embodimentcan generate a time varying Q_(M) on-time in high input voltage mode forboth V_(DS) switching loss reduction and system switching frequencyjitter.

FIG. 47 is a simplified flowchart that illustrates a method forcontrolling a power converter that embodies certain aspects of thisinvention. As shown in FIG. 47, method 4700 for controlling a powerconverter includes, at 4710, turning on a power switch to maintain adesired output voltage, wherein the power switch is coupled to a primarywinding to control a primary current flow and the output voltage isprovided by a secondary winding. Method 4700 also includes, at 4720,adding a capacitance in parallel to the power switch at a timedetermined by a magnitude of an input voltage to the power converter.Here, the timing of adding the capacitance depends on the operation modeof the power controller. In a low input voltage mode, at 4730, thecapacitance is added in a demagnetization-time during which thesecondary winding discharges. In a high input voltage mode, at 4740, thecapacitance is added in a discontinuous time.

In the examples described above, adding the capacitance in parallel tothe power switch comprises turning on a modulation switch that iscoupled in series with a capacitor. The capacitor and the modulationswitch are coupled in parallel with the power switch.

In another example described above, in the low input voltage mode,adding the capacitance comprises turning on the modulation switch aftera pre-set delay time after the power switch is turned off. In the highinput voltage mode, adding the capacitance includes turning on themodulation switch at a first peak point in a resonant waveform in thediscontinuous time after a blanking time, and turning off the modulationswitch after a time period based on a time-varying function to vary aturn-on-time of the modulation switch.

What is claimed is:
 1. A power converter, comprising: a transformerhaving a primary winding for coupling to an input voltage, a secondarywinding providing an output voltage of the power converter, and asensing circuit for providing a sensing signal through auxiliarywinding, a power switch for coupling to the primary winding of the powerconverter to control a primary current flow; a capacitor and amodulation switch coupled in parallel to the power switch, themodulation switch being coupled in series with the capacitor; and acontrol circuit, comprising: a power controller for turning on the powerswitch to maintain a desired output voltage; a mode selection circuitfor comparing the input voltage with a reference voltage and providing amode selection signal that indicates a high input voltage mode if theinput voltage is higher than the reference voltage and a low inputvoltage mode if the input voltage is lower than the reference voltage;and a modulation controller configured to turn on the modulation switchto activate the capacitor according to the mode selection signal,wherein: in the low input voltage mode, the modulation switch is turnedon in a demagnetization-time during which the secondary windingdischarges; and in the high input voltage mode, the modulation switch isturned on in a discontinuous time during which no current flows in thetransformer and the sensing signal is characterized by a resonantwaveform.
 2. The power converter of claim 1, wherein the modulationcontroller comprises a delay circuit and a one-shot circuit.
 3. Thepower converter of claim 2, wherein, in the low input voltage mode, themodulation controller is configured to turn on the modulation switchafter a pre-set delay time after the power switch is turned off in everyswitching cycle of the power converter.
 4. The power converter of claim2, wherein, in the low input voltage mode, the modulation controller isconfigured to turn on the modulation switch after a pre-set delay timeafter the power switch is turned off once in multiple switching cyclesof the power converter.
 5. The power converter of claim 1, wherein thecontrol circuit further comprises: a valley detector for detectingvalley points in the resonant waveform in the sensing signal during thediscontinuous time; and a peak detector for detecting peak points in theresonant waveform in the sensing signal during the discontinuous time.6. The power converter of claim 5, wherein, in the high input voltagemode, the modulation controller is configured to: turn on the modulationswitch at a first peak point in the resonant waveform after a blankingtime; and turn off the modulation switch after a time period based on atime-varying function to vary a turn-on-time of the modulation switch.7. The power converter of claim 6, wherein the power controller isconfigured to turn on the power switch at a valley point of the resonantwaveform after a preset off time of the modulation switch.
 8. The powerconverter of claim 7, wherein the power controller is configured to turnoff the power switch based on a constant on-time, a feedback signal, ora peak primary current sense signal.
 9. The power converter of claim 1,wherein the modulation controller varies a turn-on-time of themodulation switch to add a frequency jitter to a switching frequency ofthe power converter.
 10. A control circuit for a power converter,comprising: a power controller for turning on a power switch to maintaina desired output voltage, wherein the power switch is coupled to aprimary winding of the power converter to control a primary currentflow, and the output voltage is provided by a secondary winding, and asensing circuit provides a sensing signal through an auxiliary winding;and a modulation controller configured to activate a modulationcapacitance coupled in parallel to the power switch, wherein: in a lowinput voltage mode, the modulation switch is turned on in ademagnetization-time during which the secondary winding discharges; andin a high input voltage mode, the modulation switch is turned on in adiscontinuous time.
 11. The control circuit of claim 10, wherein themodulation controller is configured to turn on a modulation switch thatis coupled in series with a capacitor, and wherein the capacitor and themodulation switch are coupled in parallel with the power switch.
 12. Thecontrol circuit of claim 11, wherein, in the low input voltage mode, themodulation controller is configured to turn on the modulation switchafter a pre-set delay time after the power switch is turned off.
 13. Thecontrol circuit of claim 12, wherein the modulation controller comprisesa delay circuit and a one-shot circuit.
 14. The control circuit of claim11, wherein the control circuit further comprises: a valley detector forproviding valley pulse signals at valley points in a resonant waveformin the sensing signal during the discontinuous time; and a peak detectorfor providing peak pulse signals at peak points in a resonant waveformin the sensing signal during the discontinuous time.
 15. The controlcircuit of claim 14, wherein, in the high input voltage mode, themodulation controller is configured to: turn on the modulation switch ata first peak point in the resonant waveform after a blanking time; andturn off the modulation switch after a time period based on atime-varying function to vary a turn-on-time of the modulation switch.16. The control circuit of claim 15, wherein the modulation controlleris coupled to the valley detector, the peak detector, and an inputvoltage magnitude detector to provide a frequency jitter that includes afirst portion that varies with the input voltage of the power converterand a second portion that is a time-varying function.
 17. A method forcontrolling a power converter, comprising: turning on a power switch tomaintain a desired output voltage, wherein the power switch is coupledto a primary winding to control a primary current flow and the outputvoltage is provided by a secondary winding; and adding a capacitance inparallel to the power switch at a time determined by a magnitude of aninput voltage to the power converter, wherein: in a low input voltagemode, the capacitance is added in a demagnetization-time during whichthe secondary winding discharges; and in a high input voltage mode, thecapacitance is added in a discontinuous time.
 18. The method of claim17, wherein adding the capacitance in parallel to the power switchcomprises turning on a modulation switch that is coupled in series witha capacitor, wherein the capacitor and the modulation switch are coupledin parallel with the power switch.
 19. The method of claim 18, wherein,in the low input voltage mode, adding the capacitance comprises turningon the modulation switch after a pre-set delay time after the powerswitch is turned off.
 20. The method of claim 18, wherein, in the highinput voltage mode, adding the capacitance comprises: turning on themodulation switch at a first peak point in a resonant waveform in thediscontinuous time after a blanking time; and turning off the modulationswitch after a time period based on a time-varying function to vary aturn-on-time of the modulation switch.